Dispersants are usually applied by airplane or helicopter, but also can be applied by boat. To increase the chances that an application will be effective, spill responders try to choose the best combination of dispersant droplet size, concentration, and rate of application. They also try to carefully target dispersant applications, sometimes even using spotter planes that use infrared detectors to locate spilled oil as precisely as possible.
In addition, you can watch a U.S. Coast Guard video showing the application of dispersant by aircraft: Dispersant Deployed Off Galveston. The video was released by the Unified Command responding to the tank ship Krymsk incident. In the video, a DC3 aircraft deploys dispersant to an oil sheen 40 miles off of Galveston, Texas on Oct. 21, 2009, after an estimated 18,000 gallons of fuel oil were spilled from the Krymsk on Oct. 20. The Unified Command decided to deploy dispersant after determining that on-water oil recovery methods would not work due to weather conditions.